The present invention relates to a method for the suspension polymerization of vinyl chloride monomer in an aqueous medium in the presence of a monomer-soluble polymerization initiator or, more particularly, the invention relates to an improvement in the suspension polymerization of vinyl chloride monomer, according to which a high quality polyvinyl chloride resin product can be obtained with an outstandingly high bulk density and extremely small number of fish eyes.
As is well known, the largest part of polyvinyl chloride products is produced with high productivity by the method of the suspension polymerization of the monomer in an aqueous polymerization medium in the presence of a monomer-soluble polymerization initiator. The polyvinyl chloride resin products are in general required to have a bulk density as high as possible from the standpoint of obtaining a high productivity or, in other words, an extrudable amount of the resin in a unit time in the fabrication of rigid shaped articles by the technique of extrusion molding.
It is well known that, in the suspension polymerization of vinyl chloride in an aquesous medium, the bulk density of the resin product is determined depending usually on the type of the dispersing agent and the condition of agitation of the polymerization mixture during the polymerization reaction. Accordingly, various proposals have been made hitherto in these respects with an object to obtain a polyvinyl chloride resin product having an increased bulk density although the prior art methods are not quite satisfactory due to the problem that the resin product obtained by the method has a decreased porosity and contains fish eyes in an increased number.
The above mentioned porosity of a polyvinyl chloride resin is another important quality factor from the standpoint of the behavior of the resin for gelation and absorptivity of plasticizers in the process of molding and it is also known that addition of a surface active agent to the polymerization mixture has an effect of increasing the porosity of the resin product. A problem in this method is, however, the decrease in the bulk density of the resin product.
Thus, a contradictory relationship exists between the increase in the bulk density and increase in the porosity of a polyvinyl chloride resin product and it has been generally understood that it is extremely difficult to obtain compatibility between these two requirements. In recent years, proposals have been made in this regard according to which the suspension polymerization of vinyl chloride is performed in an aqueous medium containing a combination of a certain water-insoluble crosslinked polymer containing carboxyl groups and a non-ionic surface active agent (see, for example, Japanese Patent Kohyo 57-500614 and 57-500650). This method is, however, not sufficiently effective in respect of the requirements for an increased porosity and decreased number of fish eyes though not quite insignificantly effective in increasing the bulk density of the polyvinyl chloride resin product. In particular, the number of fish eyes is a very important quality parameter of a polyvinyl chloride resin product to be improved by all means due to the direct adverse influences thereof on the quality of the shaped articles molded of the resin product.